I am looking for a macro lens to add to my collection. I'm interested in getting close up photos of nearly everything, but I find macro of insects really interesting since it unlocks a totally new world to explore.

My main concern is scaring the small bugs, so I've been looking for macro lenses with a decent reach.

The Sigma 150mm is supposed to have better build quality, but some people complain that the coating of the lens can crack and fall off.
The Tamron 180mm is said to have ok build quality, handles Chromatic Abberation a tad better (not field relevant) and has the extra reach.

Sigma 150mm has a faster autofocus(HSM) while Tamron is using the old screw-focus thingy but I believe I'll be using manual focus for macro photos.

So, back to my main concern. Minimum working distance.
Sigma 150mm has about 20cm minimum working distance while Tamron 180mm has about 25cm.
*measured from the tip of the lens*

Anyone has any experience using these lenses and care to share their thoughts? Im new to macro photography, so Im not sure if the minimal working distance is _that_ important. I guess I can get a pretty good close up photo even tho I'm not at 1:1?

depends on the focal range one needs...you should test them.
I would chose the Sigma because I wouldn't want anything above that range for macro purposes. But I never tried them,it's about preference after all.

I have been using the Siga 150mm macro for a year now. I have to say that it is extremely sharp and also very well built. It is the premium line of the sigma lenses. The coating is fine and I have not had any problems with it.

Image quality is simply superb, with excellent sharpness and clarity, and not just for macro. I have not found anything to complain about in that department. Build quality is also solid. I dropped it on a tile floor, maybe from 2-3 feet high. My heart almost broke, but nothing happened to the lens fortunately. It has been solid.

I was also considering many different macro lenses including the canon 100mm and finally settled on the sigma 150mm. It gives good working distance and is also at the range where it is more versatile than the 180mm. I would recommend it without any hesitation.

All the best with your choice. I can answer questions about the lens if you have any.

20 cm or 25 cm working distance from the front-lens (w/o lens-hood) doesn't make much difference in my book.
But I can vouch for the superior image quality of the Sigma 150/2.8 non-OS as documented in my review here.
If you need perhaps not 1:1 but "only" 1:3 magnification and a much larger working distance I'd try looking for one of those used Sigma 300mm f/4.0 APO macro lenses which can go down to 1:3 with a front-lens to subject distance of 1.3m! (See my review there). On of my fav shots with that lens is:
Dragonfly 16720 by Thomas, on Flickr
(this one is not cropped)

Another alternative could be the use of a 2 diopter close-up filter on any bog-standard 70-300mm zoom. You get down to a magnifiction of at least 1:1.6 and have a working distance of 50cm!
The Nikon 70-300 VR is said to be working perfectly with a Canon 500D filter...